Resilient floor coverings are well-known and are described for example in an article entitled "Flooring Materials" in Encyclopaedia of Polymer Science and Engineering, Wiley-Interscience, 3rd edition, Volume 7 (1987), pages 233-247, and in an article entitled "Floor Coverings" in Ullmann's Encyclopaedia of Industrial Chemistry, VCH Publishing, 5th edition, Volume All (1988), pages 270-274. Resilient floor coverings are commonly composite laminates constructed from a number of plastics film layers, each layer being specially formulated for a particular duty. Thus, the topmost film, which is commonly called the wear layer, is formulated for good resistance to abrasion and wear. The wear layer may be transparent, so that the coloring or patterning of one or more lower films can be seen through it, in which case it is commonly called a clear wear layer. Such a clear wear layer generally overlies a pigmented film which is solid in color, for example white. Such a pigmented film is commonly called a face ply. A printed patterned film is generally interposed between the clear wear layer and the face ply, the presence of the face ply serving to enhance the visual appearance of the pattern as seen through the wear layer. The undermost film of the composite is commonly referred to as the backing layer and is often pigmented black. It may be embossed on the underside with a pattern, produced for example by pressing against a fabric belt, to provide good adhesion to the underlying floor. Alternatively, the floor covering may comprise a backing fabric adhered to the underside of the backing layer. In the construction known as cushion flooring the composite may contain a foam layer between the face ply and the undermost backing. The solid backing films, which include the face ply and the backing layer, are generally highly-filled thermoplastic materials. The films which lie between the wear layer and the backing may be referred to as interior films. The compositions of all the layers in the composite must be balanced to ensure that the floor covering has the correct balance of properties; for example insulating and sound-absorbing properties, and in particular freedom from a tendency to curl. Economic factors must also be borne in mind.
Resilient floor coverings based on plasticised PVC, which are commonly known as vinyl sheet and tile flooring, have enjoyed considerable commercial success but suffer from a number of disadvantages. PVC is slightly yellow and tends to become more yellow on exposure to the amounts of UV light commonly occurring in interior environments. The clarity of clear PVC wear layers and the visual appearance of patterned printed layers seen through such wear layers are not as great as could be desired. The resistance of vinyl floor coverings to wear, abrasion, scratching and scuffing is not as great as could be desired. The use of vinyl floor coverings has been objected to on environmental grounds, both in that they contain the chlorinated polymer PVC and in that they contain volatile organic compounds such as plasticizers. Proposals have been made for the manufacture of resilient floor coverings which overcome some of these disadvantages, in particular by reason of being based on thermoplastic polymer compositions which are essentially chlorine-free, as described for example in WO-A-95/08593.
In the manufacture of vinyl flooring, films, including the solid backing films, are commonly made by calendering techniques.
Calendering is well-suited to the manufacture of highly-filled vinyl films. In contrast, many of the polymers suitable for the manufacture of chlorine-free resilient floor coverings can be extruded, but cannot readily be calendered. The extrusion of highly-filled films of such polymers has been found to present practical difficulties. Polymer, often enriched with filler, tends to accumulate on the outer surface of the lips of the extrusion die in the phenomenon known as die-lip build-up. The accumulated polymer tends to become degraded as a consequence of remaining exposed at the hot exterior surfaces of the die lips. The accumulated polymer tends from time to time to break away from the lips of the die, whereupon it may become embedded in the surface of the film, thereby degrading film quality. The minimum proportion of filler at which this effect occurs depends to some extent on the nature of the composition, including the type of polymer and filler employed, and on the extrusion conditions, but we have determined that it is often of the order of 30 or 40 percent by weight for spherical fillers or of the order of 20 or 25 percent for plate-like fillers. It is an object of the invention to provide a method of manufacturing highly-filled extruded films for use as solid backing films in resilient flooring coverings which does not suffer from the problem of die-lip build-up.
EP-A-228041 describes a coextruded multilayer material having a thermoplastic core layer with at least one inorganic or organic constituent dispersed therein, sandwiched between thermoplastic outer layers substantially free of such particles. This form of construction serves to reduce the tendency of colorants, especially organic dyes, to migrate to the surface of a monolayer extruded article during and after the extrusion process, which may result in damage to the extruder, cross-contamination and color defects. It also serves to reduce the tendency of reinforcing agents to break through the surface of a monolayer extruded article during extrusion, which may result in poor quality product and ultimately in damage to the extruder. The total amount of inorganic or organic constituents generally ranges from about 1 to about 40 percent by total weight of the multilayer material.
EP-A-347745 describes a coextruded multilayer film structure containing a high percentage of fillers. The fillers are concentrated in a filler-containing layer which amounts to no more than about 5 to about 20 percent, preferably no more than about 10 percent, of the thickness of the total multilayer film. The filler-containing layer may contain about 15 to about 60 percent by weight filler. The filler-containing layer may be joined to a relatively thick base layer or may be sandwiched between two such relatively thick base layers. The base layers may contain 1 to 15 percent by weight conventional whiteners such as titanium dioxide or calcium carbonate. The base layer or layers serve to support the filler-containing layer, which is brittle and easily torn.